CH Navigation & Nautical Chart Production 1 — Questions and Answers
Question 1: What is the primary horizontal datum used for nautical charts produced by NOAA in the United States?
- NAD 27
- NAD 83 / WGS 84 (Correct answer)
- ED 50
- ITRF 2014
Correct answer: NAD 83 / WGS 84
NOAA nautical charts use NAD 83, which is nearly identical to WGS 84, as the horizontal reference datum for all positional information.
Question 2: On a nautical chart, a sounding value shown in italics typically indicates:
- A depth measured by lead-line rather than echo sounder
- A depth that is less reliable or obtained from an older survey (Correct answer)
- A reef or rock awash at chart datum
- A depth corrected for tide at time of survey
Correct answer: A depth that is less reliable or obtained from an older survey
Italic soundings on NOAA charts indicate depths that are of lesser reliability, often sourced from historical or unverified surveys.
Question 3: What does the chart symbol 'Wk' followed by a dashed outline indicate on a nautical chart?
- A waypoint for recommended anchorage
- A wreck whose position is approximate or unsurveyed (Correct answer)
- A water intake structure
- A well-surveyed wreck cleared to a known depth
Correct answer: A wreck whose position is approximate or unsurveyed
A dashed outline around a wreck symbol indicates the wreck's position is approximate and may not have been accurately surveyed.
Question 4: The Mercator projection used for most nautical charts preserves which property?
- Equal area across all latitudes
- True compass bearings (rhumb lines appear as straight lines) (Correct answer)
- Great circle routes as straight lines
- Accurate representation of polar regions
Correct answer: True compass bearings (rhumb lines appear as straight lines)
The Mercator projection is conformal, preserving local angles so that rhumb lines (constant compass-bearing courses) plot as straight lines, which is essential for navigation.
Question 5: Which vertical datum is used as the reference surface for depths (soundings) on NOAA nautical charts in US waters?
- Mean Higher High Water (MHHW)
- Mean Lower Low Water (MLLW) (Correct answer)
- Mean Sea Level (MSL)
- National Geodetic Vertical Datum 1929 (NGVD 29)
Correct answer: Mean Lower Low Water (MLLW)
NOAA uses MLLW as the chart sounding datum because it represents approximately the lowest water level, providing conservative (safe) depth values for mariners.
Question 6: The term 'Least Depth' on a chart refers to:
- The shallowest depth in a channel approach
- The minimum depth over a specific feature such as a shoal or wreck (Correct answer)
- The depth at the channel centerline during low tide
- The depth below which anchoring is prohibited
Correct answer: The minimum depth over a specific feature such as a shoal or wreck
Least depth denotes the minimum sounding value recorded over a specific hazard or feature, representing the critical clearance depth for navigation.
CH Navigation & Nautical Chart Production 1
What is the primary horizontal datum used for nautical charts produced by NOAA in the United States?